The invention relates to a polymer-supported carbonylation catalyst and its use in a process for preparing organic carboxylic acids or anhydrides having n +1 carbon atoms.
The invention relates also to a process for preparing organic carboxylic acids or anhydrides having n +1 carbon atoms by reacting, in the presence of the above-mentioned carbonylation catalyst, an alcohol having n carbon atoms, an ether having 2n carbon atoms or an ester formed from said alcohol and acid with carbon monoxide. In particular, the invention relates to a process for preparing acetic acid by reacting methanol with carbon monoxide in the presence of said carbonylation catalyst.
The most commonly known process for preparing acetic acid up to date has been the process described by Paulik et al. in U.S. Pat. Nos. 3,769,329 and 4,690,912. That process comprised synthesizing acetic acid through a carbonylation reaction of carbon monoxide and methanol in the presence of a rhodium catalyst at a temperature of 180xc2x0 C., and a carbon monoxide pressure of 35-70kg/cm2 as well as using methyl iodide as promoter. These patents disclosed that the most effective solvent for the production reaction of acetic acid was the product, acetic acid, itself. Its main advantages were that the catalyst had very high conversion and selectivity ( greater than 95%), had a relatively long service life, and could be recycled almost completely to the reactor. However, water content in those reaction system should be maintained at least higher than 14-15wt. % in order to prevent the rhodium catalyst from precipitation and keep a relatively high reaction rate. Such high water content would increase the cost of equipment used in the purification process and consume considerable energy that it becomes so-called a xe2x80x9chigh water contentxe2x80x9d carbonylation process.
Thereafter, there have been a number of patents proposing improved methods with respect to this acetic acid process. The main objective of those patents was aimed at increasing the stability of the rhodium catalyst under low water content ( less than 14 wt. %) in order to alleviate the corrosion problem of moisture reaction to the equipment and also reduce largely the energy consumption during isolation and purification of the product. Namely, they were developed toward a xe2x80x9clow water contentxe2x80x9d carbonylation process. Their main approaches comprised of (1) incorporating inorganic or organic salts as additives in the reaction medium, (2) using supported catalyst by combing the rhodium catalyst with a polymer, active carbon or ion exchange resin, and (3) using both of (1) and (2).
U.S. Pat. No. 4,733,006 disclosed the use of an inorganic salt XOAc (X=Li+,Na+,K+) as additives. However, it did not teach the effect of these inorganic salts on the reaction rate throughout the entire disclosure. EP 55618 disclosed a technique to reduce the precipitation of the rhodium catalyst due to low water content during the carbonylation of methanol by adding an organic catalyst stabilizer in the reaction solution. Stabilizers used in that patent comprised several types of organic compounds containing, concurrently or individually, one or more nitrogen or phosphorus atom, or carboxyl group (COOH):
1. N,N,N1,N1-btetramethyl-o-phenylenediamine and 2,31-dipyridyl
2. HOOC-Y1-COOH and 
where: Y1=(CX1X2)m,m=2-10 Y2,Y3,Y4,Y5=(CX1X2)nn=2-10 
Where:
R1,R2,R4, and R5 is an alkyl or an aralkyl having 1-20 carbon atoms;
R1,R2,R4,5 is an alkyl or an alkaryl having 6-20 carbon atoms
R3is a polymethylene group having 1-3 carbon atoms.
U.S. Pat. No. 5,001,259 described the use of inorganic iodides LiI as the stabilizer for the rhodium catalyst in the carbonylation of methanol to improve the precipitation of the rhodium catalyst under low water content, and obtained a reaction rate almost equal to that under high water content (14 wt. %). The same patent used also a quaternary ammonium salt, N-methyl-picolinium iodide, under low water content to increase the reaction rate of carbonylation. However, according to the result of the experiment, the N-methyl-picolinium iodide tended to form a hardly soluble complex with Rh that precipitated from the reaction solution. The nitrogen-containing compound, N-methylimidazole, mentioned in EP 1538341 tended also to form a hardly soluble complex with Rh that precipitated from the reaction solution of carbonylation of methanol.
In U.S. Pat. No. 5,442,107, six types of heterocyclic nitrogen compounds were employed as the catalyst stabilizer for the carbonylation of methanol under low water content:
1. 2-ethyl-4-methylimidazole
2. 4-methylimidazole
3. 4-tert-butylpyridine
4. 2-hydroxypyridine
5. 3-hydroxypynidine
6. 4-hydroxypyridine.
However, this patent did not disclose the effect of the additive used on the reaction rate under low water content. No alkyl pyridine was mentioned in that patent. Further, those catalyst stabilizers used in that patent were similar to those organic compounds mentioned in the prior art, i.e., picoline and N-methylimidazole, in that they tended to form a hardly soluble complex with Rh which precipitated from the reaction solution of carbonylation of methanol under low water content. These suggest that, if OH and tert-butyl were present on pyridine, there would be a significant effect of reducing the precipitation of the rhodium catalyst from the reaction solution of carbonylation of methanol under low water content. On the other hand, if no substituent was on pyridine or the substituent was a methyl group, such effect would be insignificant. Furthermore, the prior art has not mentioned or indicated that other pyridine derivatives having substituents other than OH or alkyl had the effect of reducing precipitation of the rhodium catalyst during the carbonylation of methanol under low water content.
Moreover, techniques that used complexes having a supported structure formed by coordinating an organic polymer with rhodium to prevent its precipitation in the reaction system under low water content and to increase further the concentration of the rhodium catalyst and hence the reaction rate were proposed. For example, Webber et al. described in Journal of Molecular Catalyst, 3 (1977/78) 1-9, that a polymer with two functional group was used in a carbonylation reaction. In the practical application of such polymer, however, there were still some problems such as lost of rhodium as well as their stability in the industrial application.
A rhodium complex formed by supporting rhodium with a 2,4-divinylpyridine was used in EP 277824 to perform a heterogeneous carbonylation reaction. In such a system, however, not only the activity of the catalyst was decreased significantly, but also the pyrolysis of the polymer itself or the reaction medium-mediated chemical decomposition of the polymer would resulted in the stripping of the rhodium off the polymer and the complicating of the purification system. Accordingly, such system has not been used in any industrial process.
As described in Journal of Catalyst, 40,255-267 (1975), a copolymer of a styrene having a diphenylphosphinyl group and a divinylbenzene was used as the support of rhodium in liquid and gas phase carbonylation reactions. ROC Patent Nos. 080618 and 094905 described the use of a series of phosphorus-containing liquid such as PPh3 and Ph3PCH2CH2P(O)Ph2. However, in such a reaction system, dissociation of the polymer from the rhodium atom might cause the precipitation of the rhodium. Further, during the reaction, addition of excess of triphenylphosphine was necessary in order to keep so-claimed high catalytic activity.
Inorganic Chemistry, 20,641-644(1981) described reacting ion exchange resin such as Bio Rex 9 Dowex 1-X8 and the like or a copolymer of styrene and 4-vinylpyridine alkylated with methyl iodide with tetracarbonyl dichloro dirhodium or rhodium trichloride hydrate to form a heterogeneous catalyst. Although the author claimed that it could have a reaction effect comparable to that of a homogeneous catalyst, this was suggested by only an experimental result at low temperature of 120xc2x0 C. and low pressure of 80-100 psi. In addition, when reacted in a liquid phase, a great amount of ion exchange resin must be maintained to proceed the reaction efficiently.
U.S. Pat. Nos. 5,281,395 and 5,466,874 used poly2-pyrrolidone crosslinked with divinylbenzene as a support for rhodium catalyst and carried out carbonylation of methanol under low water content to produce acetic acid or anhydride. As the experimental result indicated, although the rhodium complex catalust stabilized by such polymer could be funtioned at low water content, it was true only on the provision that a high cocentration of rhodium complex catalyst (e.g.,an alkyl iodides having an equalor even more carbon number) and an inorganic metal iodide must be present. It was undesirable in that excess metal iodides might lead to a corrosoin problem on the equipment used. Further, copolymers consist of 2-pyrrolidone had problems on the probability for formation of rhodium complex therewith and on the stability for long-time operation.
Accordingly, in view of overcoming the above-mentioned disadvantages of the prior art, one object of the invention is to provide a novel polymer-supported catalyst by supporting rhodium catalyst with a polymer.
Another object of the invention is to provide a process for preparing organic carboxylic acid or anhydride having n +1 carbon atoms by carbonylating with carbon monoxide on alcohols having n carbon atoms, ethers having 2n carbon atoms or esters formed from an alcohol with an acid, in the presence of the above-said polymer-supported catalyst system.
Still another object of the invention is to provide a process for preparing acetic acid by reacting methanol with carbon monoxide in the presence of the above-mentioned polymer-supported catalyst system.
The polymer-supported catalyst according to the invention has a functional structure of formula 
wherein:
A: is an unsaturated aliphatic or aromatic radical containing nitrogen, phosphorus or sulfur atom;
B: is A or an unsaturated aliphatic or aromatic alcohol, ether, aldehyde, ketone or carboxylic ester group containing oxygen, sulfur or phosphorus atom;
M: is rhodium, iridium or group VIII transitional metal;
L: is a carbonyl, triphenylphosphine or other ligand;
Y: is a halogen, tetraphenylboron, tetrafluoroboron, acetic acid, carbonate, bicarbonate or thiocyanate anionic radical;
x is 1 to 2;
n is 1-5;
m is 2-300.
In the process for preparing the above-said catalyst according to the invention, two type of monomers are used, namely, 2-vinyl pyridine and methyl acrylate, in an appropriate ratio in solution polymerization to form a copolymer. The copolymer thus obtained is used then to coordinate with rhodium compound such as rhodium chloride, rhodium iodide, rhodium acetate and the like to form polymer-supported rhodium catalyst. Before the coordination, those rhodium compounds must be selectively treated to reduce rhodium(III) into rhodium(I). This type of catalyst has advantages in that its stable structure can impart such catalyst system excellent thermal stability and chemical stability under the operation conditions of carbonylation reaction.
In another aspect of the invention, a process for preparing organic carboxylic acid or anhydride having n +1 carbon atoms by carbonylating with carbon monoxide on alcohols having n carbon atoms, ethers having 2n carbon atoms or esters formed from an alcohol with an acid, in the presence of the above-said polymer-supported catalyst system is provided. In the presence of such polymer-supported rhodium catalyst, when the reaction system is operated under a insufficient carbon monoxide partial pressure and an elevated temperature, due to the stability of its structure, the rhodium metal can be protected from precipitating in the reaction medium. Further, its catalytic activity can be maintained without adding excess hydroiodic acid such that the corrosion problem of equipment due to the addition of excess hydroiodic acid can be reduced greatly and hence save a lot of equipment investment.
In addition, in traditional xe2x80x9chigh water contentxe2x80x9d process for preparing acetic acid from methanol and carbon monoxide, water content of up to 14xcx9c15 wt % must be present in the reaction medium in order to increase the solubility of rhodium catalyst and enhance its catalytic activity. On the contrary, in the catalyst system according to the invention, the rhodium catalyst is in polymer-supported form, water content can no longer have a significant effect on the precipitation of the rhodium catalyst. Further, the high catalytic activity of the catalyst system according to the invention can overcome successfully the decrease of reaction rate under low water content associated with the traditional process and hence retain a relatively excellent reaction performance. As the water content is lowered, production capacity of a reactor having a same size can be increased. Further, the corrosion problem caused by moisture reaction to the reaction system due to a large amount of water in the reaction system can be avoided. Moreover, the elimination of the load on the separation equipment can have a somewhat benefit for the investment efficacy. Particularly, as the water content is lower than a given ratio, esters in the reaction system can be reduced as the reaction proceeds and thereby acid or anhydride can be formed such that the utility and yield of the process can be extended relatively.
In the practice of the process according to the invention, alcohol, ester of the alcohol with the carboxylic acid as the desired product, or the carboxylic acid itself, together with carbon monoxide, polymer-supported rhodium catalyst, organic iodine derivative, a selected ratio of hydroiodic acid, inorganic halide, stabilizer for catalyst and water are charged into a carbonylation reactor. Among them, the polymer-supported rhodium catalyst, organic iodine derivative, hydroiodic acid, inorganic halide, metal acetate, and the stabilizer for catalyst will not be consumed during the reaction and will return continuously to the reactor through a flashing tank or a separation and purification apparatus. In correspondence with continuous addition of various raw materials into the carbonylation reactor, the effluent product of the system according to the invention contains the carboxylic acid product and the corresponding anhydride, the rhodium catalyst and the iodine derivative. Further, in the reaction system according to the invention, when a steady sate of a homogeneous or heterogeneous phase is maintained in the carbonylation reactor, the reaction medium can have a given amount of polymer-supported rhodium catalyst, organic iodine promoter, alcohol, ester of the carboxylic acid and the carboxylic acid itself, as well as a selected ratio of hydroiodic acid, inorganic halide and water. The carbonylation reactor contains actually, however, only small amount of free alcohol, since esterification reaction of alcohol with acid proceeds quiet rapidly.
For the process according to the invention, the composition of various reaction components in the homogeneous or heterogeneous reaction medium at a steady state is preferably within the range listed in Table 1:
In another aspect of the invention, a process for preparing acetic acid by carbonylation of methanol in the presence of the instant polymer-supported rhodium catalyst, the preferable composition may be as follows:
methyl acetate, 0.1xcx9c5 wt %;
methyl iodide, 10xcx9c30 wt %;
polymer-supported rhodium catalyst, 500xcx9c2000 ppm;
water, 1xcx9c10 wt %;
hydroiodic acid, 3xcx9c20 wt %;
inorganic halide or acetate, 3xcx9c20 wt %; and
acetic acid of the remainder.
In the process according to the invention, temperature of the carbonylation reactor is suitably kept at 100xcx9c250xc2x0 C., and the higher the temperature, the reaction rate is faster. Preferred temperature is in the range of 160xcx9c220xc2x0 C. The pressure of carbon monoxide is maintained at 10xcx9c200 atmospheric pressure, preferably in the range of 20xcx9c60 atmospheric pressure.